High-temperature direct-contact thermal energy storage using phase-change media

ABSTRACT

A high-temperature direct-contact thermal energy storage element for use in a system for storage and retrieval of thermal energy in the range of about 400° to about 2000° F. The thermal energy is directly stored, without heat exchange tubes in composite latent/sensible heat thermal energy storage media utilizing the heat of fusion and high-temperature stability of alkaline metal and alkaline earth carbonates, chlorides, nitrates, nitrites, fluorides, hydroxides, sulfates, and mixtures thereof maintained within a porous support-structure material which itself is capable of storage as sensible heat. The thermal energy storage according to the invention may be effectively utilized for storage of thermal energy derived from solar, industrial waste, process heat, and high-temperature gas reactor energy sources and retrieved for a wide variety of uses such as combustion air preheating, drying, space heating, heating of process gases, and the like.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to high-temperature direct-contact thermal energystorage. The thermal energy stored according to this invention is in thetemperature range of about 400° to 2000° F. and is directly stored,without the necessity of heat exchange tubes to provide containment andheat transfer surface, in composite latent/sensible heat thermal energystorage media utilizing the heat of fusion and high temperaturestability of alkali metal and alkaline earth carbonates, chlorides,nitrates, nitrites, fluorides, hydroxides, sulfates and mixtures thereofmaintained within a porous storage-support material which itself iscapable of storage of sensible heat. Various mixtures of alkali metaland alkaline earth salts may be used to obtain desired thermal andphysical properties for storage of thermal energy derived from solarenergy sources, industrial waste and process heat, high temperature gasreactors and the like.

2. Description of the Prior Art

There have been many prior attempts to store thermal energy utilizingthe heat of fusion in aqueous-hydrate systems. For example, U.S. Pat.No. 3,986,969 teaches the use of a heat of fusion material plusattapulgite clay as a homogenizing agent; U.S. Pat. No. 1,894,775teaches various latent heat of fusion storage chemicals; U.S. Pat. No.4,146,057 teaches thermal storage as latent heat of fusion by passing aclosed potassium loop and a closed steam/water loop through aluminumwhich serves to store the latent heat of fusion and is exemplary ofprior practices utilizing tube heat exchangers to provide containmentand heat transfer surface between storage media and working fluid; U.S.Pat. No. 4,223,721 teaches thermal storage by a eutectic salt packedwith a thermal insulating material such as glass fiber insulation; andJapanese Patent No. 51-96788 teaches thermal storage at about roomtemperature by hydrate reaction of Na₂ S0₄ and/or Na₂ CO₃ with a lightaggregate such as gypsum for support and prevention of deliquescence orefflorescence. U.S. Pat. No. 4,237,023 teaches an aqueous heat storagecomposition which absorbs and stores heat as it is heated above itsphase-change temperature and releases stored heat as it is cooled belowits phase-change temperature, including use of fumed silicon dioxidewhich acts as a stabilizing agent and provides prolonged heat storageefficiency. U.S. Pat. No. 3,720,198 teaches thermal storage by heat offusion with seed crystals in the thermal storage material to preventchange of distribution during the melting phase. The prior art hydratesystems have experienced problems with supercooling and phase separationwhich the last two patents referred to seek to overcome.

SUMMARY OF THE INVENTION

This invention provides a high-temperature direct-contact thermal energystorage element comprising a phase-change salt retained within a porous,sensible heat storage phase. Thermal energy is directly stored accordingto the invention at about 400° to about 2000° F. without the use ofconventional heat exchange means. The thermal performance is enhanced bythe direct contacting of working fluid and the thermal energy storageelement of this invention. Suitable working fluids for direct contactare carbon dioxide, oxygen air, hydrogen, inert gases such as helium,nitrogen, and combustion gases.

The high-temperature direct-contact thermal energy storage element ofthis invention is a composite of phase-change chemical comprising about10 to 90 volume percent and a thermal energy storage-support materialwhich comprises about 10 to 90 volume percent. This compositelatent/sensible heat thermal energy storage element utilizes the heat offusion and high temperature stability of alkali metal and alkaline earthcarbonates, chlorides, nitrates, nitrites, fluorides, hydroxides,sulfates, and mixtures thereof having a solid-liquid phase-changetemperature at about 400° to about 2000° F. The phase-change chemical isretained by capillary action within the pores of a thermal energystorage-support material which itself is capable of sensible heatstorage. Suitable thermal energy storage-support materials are metaloxides, aluminates, titanates, and zirconates having sub-micronparticles which do not substantially coarsen with thermal cycling attemperatures up to about 2000° F. and having a surface area greater thanabout 10 square meters per gram. Exemplary of suitable thermal energystorage-support materials are lithium aluminate, sodium aluminate,magnesium oxide, alumina, lithium ferrite, lithium titanate, bariumtitanate, strontium titanate, and mixtures thereof. The thermal andphysical properties of the thermal energy storage element are controlledby varying the phase-change chemical and the relative proportions ofphase-change chemical and thermal energy storage-support material.

Powders of composite material comprising phase-change chemical andthermal energy storage-support particles are prepared by dry blending orspray drying. Resulting powders are formed into suitable shapes by coldpressing, extrusion, briquetting or other suitable methods, and heatedabove the phase-change chemical melting point to densify and obtain thedesired shape.

It is an object of this invention to provide a high-temperature thermalenergy storage element comprising latent/sensible heat media and processthat overcomes many of the disadvantages of the prior art thermal energystorage elements and processes.

Accordingly, it is an object of this invention to provide ahigh-temperature direct-contact thermal energy storage elementcomprising composite latent/sensible heat media and process thateliminates the need for conventional heat exchange equipment.

It is another object of this invention to provide a high-temperaturethermal energy storage element that provides direct contact betweenstorage media and working fluid.

It is yet another object of this invention to provide a heat exchangeprocess utilizing the high-temperature direct-contact thermal energystorage element of this invention.

These and other objects, advantages and features of this invention willbecome apparent from the following description and reference to thedrawing.

BRIEF DESCRIPTION OF THE DRAWING

The FIGURE shows a simplified schematic flowsheet for a thermal energystorage and retrieval process utilizing the high-temperaturedirect-contact thermal energy storage elements of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The composite latent/sensible heat thermal energy storage media of thisinvention take advantage of the latent heat of fusion and hightemperature stability of alkali metal and alkaline earth carbonates,chlorides, nitrates, nitrites, fluorides, hydroxides, sulfates andmixtures thereof. These phase-change chemicals have a maximum operatingrange of about 400° to about 2000° F. Preferred phase-change chemicalsare alkali metal and alkaline earth carbonates, such as sodium,potassium, lithium, magnesium, calcium, strontium and barium carbonateswhich define phase-change temperatures of about 700° to about 1700° F. Adesired operating range between the temperature limits can be producedby varying the relative amounts of the individual components in aphase-change chemical mixture. The use of mixtures of phase-changechemicals is not restricted to eutectic compositions to obtain desiredthermal and physical properties. U.S. Pat. No. 3,720,198 teaches the useof seed crystals to prevent supercooling. The phase-change chemicals ofthis invention do not supercool. Hence, the need for seed crystals iseliminated. U.S. Pat. No. 3,720,198 teaches the use of metallic salthydrates which melt in their own water of crystallization and doublesalt hydrates as thermal storage substances. The use of hydratesrestricts the operating range of the thermal storage to lowtemperatures, below about 200° F. compared to the high temperature rangeof about 400° to 2000° F. for which the thermal storage of the presentinvention may be utilized.

The phase-change chemicals utilized in this invention are supportedwithin the pores of a material which also serves as sensible thermalenergy storage. The sensible thermal storage-support material needs tobe of sub-micron particle size and having a surface area greater thanabout 10 square meters per gram of storage-support material. Preferably,the finished storage-support material has a particle size about 0.05 toabout 0.5 microns and has a pore surface area about 10 to about 50square meters per gram of storage-support material. Furthermore, thesensible thermal storage-support material must not substantially coarsenwith thermal cycling at temperatures up to about 2000° F. Thestorage-support material should be substantially insoluble in andchemically inert to the phase-change chemical over the high temperaturerange used in the present process. Various metal oxides are suitable,such as lithium aluminate, sodium aluminate, magnesium oxide, alumina,lithium ferrite, lithium titanate, barium titanate, strontium titanate,and mixtures thereof. Particularly preferred are lithium aluminate,sodium aluminate, magnesium oxide, and mixtures thereof.

The sensible heat storage-support structure may be reinforced againstthermal cycle cracking by the addition of ceramic or metallicparticulates or fibers or mixtures thereof. The effective thermalconductivity can be regulated by the addition of metallic particulatesor fibers or mixtures thereof to the thermal energy storage-supportmaterial. The additive ceramic or metallic reinforcement, such asaluminum, stainless steel, Fe-Ni-Cr-Al alloys, copper-aluminum alloysmay be present in an amount up to about 5 to about 20 volume percent,based upon volume of the sensible heat storage-support material. It ispreferred to use fibers of stainless steel containing approximately 5 to10 weight percent aluminum and of a size 5 to 100 microns making upabout 5 to about 10 volume percent of the sensible heat storage-supportmaterial. The use of ground glass fibers for shape retention as taughtby U.S. Pat. No. 3,720,198 is not suitable in the thermal energy storageelement of the present invention since it will chemically react withmany of the phase-change chemicals, such as the carbonates, at theoperating temperatures.

The high temperature direct-contact thermal energy storage element ofthis invention comprises the aforementioned solid-liquid phase-changechemical supported within the pores of the aforementioned sensiblethermal storage-support material by capillary action. The hightemperature thermal energy storage element comprises about 10 to about90 volume percent solid-liquid phase-change chemical and about 10 toabout 90 volume percent storage-support material and preferably about 50to about 80 volume percent solid-liquid phase-change chemical and about20 to about 50 volume percent storage-support material. By properselection of the solid-liquid phase-change chemical storage-supportmaterial ratio, storage-support material composition and particle size(surface area to weight ratio), and solid-liquid phase-change chemicalcomposition, various shapes (pellets, briquettes, spheres) of differingheat capacities and phase-change temperatures may be fabricated. Thesethermal energy storage elements retain their shape and integrity duringrepeated melting/solidification thermal cycles at temperatures suitablefor storage of thermal energy from solar sources, industrial waste,process heat and high temperature reactor gas cooling streams. Thethermal energy may be recovered and used for a wide variety of uses suchas combustion air preheating, drying, space heating, heating of processgases, and the like.

Powders of composite material containing phase-change chemical andsensible thermal storage-support particles may be prepared by dryblending or spray drying according to methods known to the art.Resulting powders are formed by cold pressing, briquetting or extrusionand heated above the phase-change chemical melting point to densify andobtain the desired shape. Alternately, sensible storage-support bodiesof controlled porosity and pore size distribution obtained by sinteringmay be impregnated with the molten phase change chemical to produce thecomposite thermal energy storage element. Suitable shape and size forthe thermal energy storage elements of this invention are discreteshapes such as pellets, briquettes, spheres or other shapes of about 0.5to about 12 inches in their maximum overall dimension. The pelletsshould be so shaped and sized as to allow packing within a containmentvessel while providing low pressure drop working fluid passage.Alternatively, the composite media may be formed into brick-shapedelements (e.g., rectangular shapes approximately 9"×41/2"×3") or intohexagonal shapes with gas flow passages, which can be stacked in anordered arrangement. The containment vessel may be of any suitablematerial and thermally insulated either internally or externally.

An important advantage of the high-temperature thermal energy storageelement according to this invention is that it allows direct contactbetween storage media and suitable working fluids, for both thermalstorage and retrieval, thereby increasing thermal performance. Suitableworking fluids for direct contacting for thermal storage and retrievalare carbon dioxide, air, oxygen, hydrogen, inert gases such as helium orargon, nitrogen and combustion gases, and non-reactive mixtures thereof.By such direct contacting, the high-temperature thermal energy storageelement of this invention eliminates the need for conventional heatexchange means. Dry inert gases can be used in contact with compositethermal energy storage and retrieval media comprising a broad range ofphase-change materials encompassing the alkali and alkaline earthcarbonates, chlorides, nitrates, nitrites, fluorides, hydroxides,sulfates and mixtures thereof. For gases containing oxygen, carbondioxide or water vapor, the carbonate salts are generally preferredbecause of their greater high-temperature chemical stability inoxidizing environments. Composite thermal energy storage and retrievalmedia comprising carbonates are tolerant to water vapor in the gases, upto certain limits depending on temperature, carbonate composition, gascomposition and gas pressure. The carbonate eutectic 62 mol % Li₂ CO₃-38 mol % K₂ CO₃ retained within lithium aluminate support can tolerateat least 5 to 10 volume percent H₂ O in O₂ -CO₂ gas mixtures at 400° to1600° F. and 1 atmosphere pressure. The working fluid may function as astorage working fluid for thermal energy passing from a thermal energyheat source to the thermal energy storage elements in a storage mode oras a retrieval working fluid for thermal energy transfer from thethermal energy storage elements in a retrieval mode. The storage workingfluid and retrieval working fluid may be the same or may be different aslong as there is no undesired chemical reaction between them.

A high-temperature direct contact thermal energy storage and retrievalsystem according to this invention may comprise a plurality ofhigh-temperature direct contact thermal storage elements as describedabove, such as those comprising about 10 to about 90 volume percentsolid-liquid phase-change chemical having a phase-change temperature atabout 400° to about 2000° F. and selected from the group consisting ofalkali metal and alkaline earth carbonates, chlorides, nitrates,nitrites, fluorides, hydroxides, sulfates and mixtures thereof supportedwithin the pores of sensible thermal energy storage-support materialselected from the group consisting of metal oxides, aluminates,titanates and zirconates having sub-micron particle size which do notsubstantially coarsen with thermal cycling at temperatures up to about2000° F. and having a surface area greater than about 10 square metersper gram of storage support material, said storage-support materialcomprising about 10 to about 90 volume percent of said thermal storageelement; at least one containment vessel(s) for the thermal storageelements; a storage working fluid for thermal energy passing from athermal energy heat source to the storage elements by direct contactwith the phase-change chemical at a temperature higher than itsphase-change temperature and means for passage of the storage workingfluid from the heat source to contact with the phase-change chemical ina storage mode; and a retrieval working fluid for thermal energyretrieval from the storage elements by direct contact with saidphase-change chemical at a temperature lower than its phase-changetemperature and transfer to a desired use; and means for passage of theretrieval working fluid from contact with the phase-change chemical tothe desired use in a retrieval mode. It is readily apparent that whenone containment vessel is used the system must be cycled in the storageand retrieval modes. When two and more containment vessels are used, thesystem may be operated simultaneously in both the storage and retrievalmodes and on a continuous basis by proper cycling of the storage andretrieval streams to the individual containment vessels. It will bereadily apparent to one skilled in the art that a wide range oftemperatures and thermal capacities may be accommodated by varying sizesand numbers of containment vessels to obtain desired thermalperformance.

The process of thermal energy storage according to this inventioncomprises passing a storage working fluid stream in contact with aplurality of thermal energy storage elements as defined above forthermal storage, the inlet temperature of the storage working fluidbeing above the phase-change temperature of the phase-change chemical,and then passing a retrieval working fluid stream in contact with thethermal energy storage element for retrieval of thermal energy, theinlet temperature of the retrieval working fluid stream being below thephase-change temperature of the phase-change chemical, therebyincreasing the temperature of the retrieval working fluid stream.

The FIGURE illustrates in simplified schematic form one embodiment of asystem for the storage and retrieval of thermal energy utilizing thehigh-temperature direct-thermal energy storage elements of thisinvention. Thermal energy storage elements 11 are housed in a suitablecontainment vessel 12. Storage working fluid is circulated byconventional means from a high-temperature thermal energy source 14through storage working fluid conduit system 13 and brought into directcontact with the thermal energy storage elements at a temperature abovethe melting temperature of the phase-change chemical. Then a retrievalworking fluid is circulated through retrieval working fluid conduitsystem 15 and brought into direct contact with the thermal storageelements by conventional means at a temperature lower than thephase-change temperature thereby heating the retrieval working fluidwhich delivers heat to a desired use indicated by use means 16. Suitablematerials for construction, shape, size and number of containmentvessels, piping, fluid transport means and associated equipment isreadily apparent to one skilled in the art upon reading of thisdisclosure.

The following examples are set forth for specific exemplification ofpreferred embodiments of the invention and are not intended to limit theinvention in any fashion.

EXAMPLE I

Composite powder comprising 63 volume percent phase change chemicalhaving a melting point of 910° F. made up of 62 mol percent lithiumcarbonate (Li₂ CO₃) and 38 mol percent potassium carbonate (K₂ CO₃) and37 volume percent lithium aluminate (LiAlO₂) ceramic storage-supportmaterial was prepared by spray drying an aqueous slurry of γ-Al₂ O₃,LiOH.H₂ O and KOH. The spray dried powders were reacted with CO₂ andheated at 1200° F. to convert the composite powder to a mixture ofLiAlO₂ and Li₂ CO₃ -K₂ CO₃ eutectic. The LiAlO₂ particles had surfaceareas of greater than about 15 m² /g. The powders were cold-pressed intopellets measuring approximately 0.60 inch high × 0.81 inch in diameter(mass about 7.30 gram) using a steel die, at pressures of 24,490 psi.The pellets were then heated in an air furnace for an accumulated 232hours at 1027° F. and subjected to three severe thermocycles (each one atotal of about 20 minutes in duration) in which its temperature waslowered to 672° F. at a rate of about 122° F./min and raised back to1022° F. in about 15 minutes.

Examination of the pellets after these thermocycles indicated effectivesupport of the molten carbonate phase by the sub-micron sized LiAlO₂particles with only minor weight loss, some minor crack formation but nopellet fracturing and good shape retention.

EXAMPLE II

Composite powder comprising 50 weight percent phase-change chemicalsodium carbonate (Na₂ CO₃) having a melting point of 1576° F. and 50weight percent magnesium oxide (MgO) ceramic storage-support materialwas prepared by dry blending of the powders. Pellets measuringapproximately 0.5 inch high and 0.80 inch in diameter and havingmagnesium oxide surface areas greater than 16 square meters per gramwere again formed by cold-pressing in a steel die in the same manner asdescribed in Example I. The pellets were successfully thermal cycled inan air oven through the sodium carbonate melting point (once from roomtemperature to 1652° F. and back) and examined. The pellets displayedgood shape retention and integrity with no significant loss of salt.

While in the foregoing specification this invention has been describedin relation to certain preferred embodiments thereof, and many detailshave been set forth for the purpose of illustration, it will be apparentto those skilled in the art that the invention is susceptible toadditional embodiments and that certain of the details described hereincan be varied considerably without departing from the basic principlesof the invention.

We claim:
 1. A high-temperature direct-contact thermal energy storageelement for storage of thermal energy at about 400° to about 2000° F.comprising:a containment vessel housing about 10 to about 90 volumepercent solid-liquid phase-change chemical having a phase changetemperature at about 400° to about 2000° F. and selected from the groupconsisting of alkali metal and alkaline earth carbonates, chlorides,nitrates, nitrites, fluorides, hydroxides, sulfates, and mixturesthereof supported within the pores of sensible thermal energystorage-support material through which heat storage and heat retrievalfluid may be circulated selected from the group consisting of metaloxides, aluminates, titanates and zirconates, having sub-micron particlesize, the particles of which do not substantially coarsen with thermalcycling at temperatures up to about 2000° F., and having a surface areagreater than about 10 square meters per gram of storage-supportmaterial, said storage-support material comprising about 10 to about 90volume percent of said thermal storage element.
 2. The high-temperaturedirect-contact thermal energy storage element of claim 1 wherein saidsolid-liquid phase-change chemical comprises about 50 to 80 volumepercent and said storage-support material comprises about 20-50 volumepercent.
 3. The high-temperature direct-contact thermal energy storageelement of claim 2 wherein said storage-support material comprisesporous ceramic storage-support material having particle size about 0.05to about 0.5 microns and a pore surface area about 10 to about 50 squaremeters per gram of ceramic material.
 4. The high-temperaturedirect-contact thermal energy storage element of claim 2 wherein saidphase-change chemical is selected from the group consisting of sodiumcarbonate, potassium carbonate, lithium carbonate, magnesium carbonate,calcium carbonate, strontium carbonate, barium carbonate and mixturesthereof.
 5. The high-temperature direct-contact thermal energy storageelement of claim 4 wherein said phase-change temperature is about 700°to about 1700° F.
 6. The high-temperature direct-contact thermal energystorage element of claim 2 wherein said storage-support material isselected from the group consisting of lithium aluminate, sodiumaluminate, magnesium oxide, alumina, lithium ferrite, lithium titanate,barium titanate, strontium titanate, and mixtures thereof.
 7. Thehigh-temperature direct-contact thermal energy storage element of claim3 wherein said storage-support material is selected from the groupconsisting of lithium aluminate, sodium aluminate, magnesium oxide,alumina, lithium ferrite, lithium titanate, barium titanate, strontiumtitanate, and mixtures thereof.
 8. The high-temperature direct-contactthermal energy storage element of claim 3 wherein said phase-changechemical is selected from the group consisting of sodium carbonate,potassium carbonate, lithium carbonate and mixtures thereof.
 9. Thehigh-temperature direct-contact thermal energy storage element of claim8 wherein said sensible thermal storage-support material is selectedfrom the group consisting of lithium aluminate, sodium aluminate,magnesium oxide and mixtures thereof.
 10. The high-temperaturedirect-contact thermal energy storage element of claim 2 wherein saidsensible thermal storage-support material additionally comprisesreinforcement selected from the group consisting of ceramic and metallicparticulates and fibers and mixtures thereof.
 11. The high-temperaturedirect-contact thermal energy storage element of claim 10 wherein saidreinforcement comprises about 5 to about 20 volume percent, based uponthe volume of said storage-support material, of reinforcement selectedfrom the group consisting of aluminum, stainless steel, Fi-Ni-Cr-Alalloys, and copper-aluminum alloys.
 12. The high-temperaturedirect-contact thermal energy storage element of claim 2 wherein saidsensible thermal storage-support material additionally comprises athermal conductivity enhancing agent selected from metallic particulatesand fibers and mixtures thereof.
 13. The high-temperature direct-contactthermal energy storage element of claim 2 of discrete shapes havingtheir maximum overall dimension of about 0.5 inch to about 12 inches.